MEMS based inkjet print heads using a bimorph actuator comprising a silicon actuator membrane and a thin film piezo (TFP) are known in the art. For actuator performance and robustness (life-time) a low driving voltage may be crucial. Low voltage operation implies that an actuator should be able to deliver a large volume displacement per Volt [pl/V] at a given actuator compliance [pl/bar], the latter being determined by the desired acoustic design of the print head. For low voltage operation two factors are important:    1) The coupling efficiency, i.e. the required electrical energy to obtain a certain mechanical bimorph operation of the actuator membrane. The coupling efficiency may be expressed in terms of the above described volume displacement per Volt [pl/V] and the compliance of the actuator membrane [pl/bar]. The coupling efficiency is related to the thickness ratio of the thin film piezo and the actuator membrane. Optimum values of this thickness ratio depend on the basic material properties of the TFP and the actuator membrane and is approximately 1 for PZT piezo material which is a ceramic material comprising lead (Pb), zirconate (Zr) and titanate (Ti), e.g. in the following composition: Pb[ZrxTi1-x]O3, wherein 0<x<1 and a silicon actuator membrane;    2) Electric capacitance of the piezo, representing the amount of electrical energy that can be stored in the TFP for a given electric potential difference (voltage). The electric capacitance is proportional to the ratio of TFP surface area and TFP thickness.
For low voltage operation both factors should be large, which implies the use of a large area of TFP (thus having a large electric capacitance) on an actuator membrane, wherein the thickness ratio of the TFP and the actuator membrane is optimized in order to maximize the coupling efficiency. In the case of a silicon actuator membrane and PZT TPF, the actuator membrane and the TFP substantially have the same thickness.
A disadvantage of a large area thin actuator membrane is that such actuator membranes are often too compliant for a proper operation of the ink jet printing device, leading to all kinds of artifacts which may negatively influence the print quality.
Another disadvantage of such an actuator membrane is that the miniaturization of ink jet printing devices shows some unwanted restrictions (e.g. restricted maximum single pass resolution).
The compliance of the actuator membrane may be decreased by increasing the aspect ratio of the actuator membrane, i.e. increasing the membrane length, while maintaining the required surface area of the actuator membrane. In other words: the surface area of thin membranes may be increased together with increasing the aspect ratio of the actuator membrane in order to maintain a required compliance of the actuator membrane.
Following the above design strategy may lead to actuator membranes having a relatively large length, thus also requiring long pressure chambers.
Longer pressure chambers, may have a marked influence on the acoustics inside the pressure chamber (also referred to as ink channel): by actuation, an acoustic pressure response and a corresponding flow profile may be generated in a liquid present in the pressure chamber, e.g. an ink composition, enabling the liquid to be jetted out of a nozzle arranged in fluid connection with the pressure chamber. The pressure response and flow profile may depend on the properties of the liquid, such as its density and viscosity, and other dimensions of the liquid containing parts of the print head such as the depth of the pressure chamber.
In general the acoustic properties (e.g. resonance frequencies) inside the pressure chamber may be determined to a large extent by the combined (i.e. sum) compliances of the ink volume present in the pressure chamber and of the actuator membrane, i.e. the total compliance. To a certain extent, the above mentioned compliances may be interchangeable, for example if the compliancy of the ink volume is reduced (by changing the composition of the ink and/or the geometry of the pressure chamber), the compliancy of the actuator membrane(s) may be increased to the same extent, such that the total compliance and hence the acoustic properties inside the pressure chamber remain the same.
It is a disadvantage of the configuration as described above (i.e. relatively long actuator membranes positioned on relatively long pressure chambers), that the efficiency of generating the required pressure response and flow profile may decrease, e.g. due to an increased liquid volume, such that efficient operation of the ink jet printing device is not possible.
It is therefore an object of the present invention to provide an ink jet printing device that solves or at least mitigates the above stated disadvantages, the ink jet printing device thus having a robust and durable design, which may be operated at a low driving voltage, in particular below 30 V, without compromising the effective operation of the printing device and the resulting print quality.